Variable capacitors are commonly used for voltage controlled oscillator (VCO) circuits employed in communications systems and instrumentation. Advances in communications systems and instrumentation have created a need for VCOs that can operate at higher frequencies, that are less sensitive to the vagaries of temperature, that can be incorporated into integrated circuits, and that can be easily and more precisely tuned. Such VCOs require temperature stable high bandwidth capacitors whose value of capacitance can be easily predicted and controlled.
One traditional approach for developing voltage-controlled variable capacitors is to employ two-terminal varactor diodes. Varactor diodes are bipolar semiconductor devices fabricated such that there is an inverse relationship between the value of a reverse bias applied across the pn junction and the capacitance of the junction. Varactor diodes have limited application because: their capacitance response to an applied bias voltage varies with temperature; the value of capacitance as a function of the applied voltage is a complicated function; and their doping requirements differ significantly from the semiconductor technology used to fabricate the circuit thereby affecting circuit performance when varactor diodes are integrated onto a single chip with other circuit components. Also, varactor diode devices are traditionally single port devices which suffer from voltage leakage from the control voltage terminal to the output terminal affecting the value of the circuit's equivalent capacitance.
U.S. Pat. No. 4,516,041 (Quan, "Voltage Variable Capacitor") discloses a design for a voltage-controlled variable capacitor that doesn't employ varactor diodes. Quan's patent is for a one-terminal voltage-controlled variable capacitor based on the well-known Miller capacitor in which a voltage amplifier varies the value of the capacitor. While Quan's patented device does generate a capacitance that can be linearly varied with an external control voltage, the device's high input impedance limits its use in high frequency applications. In addition, the device suffers from temperature instability.
Another design approach for making voltage variable capacitors is to employ a switched capacitor network in which the effective capacitance is determined by switching capacitors in and out of the network. The capacitance value can be changed only in a discrete fashion, and such designs are complicated requiring an array of capacitors to implement, consuming large areas on an integrated chip. Therefore, such devices are expensive.
My invention overcomes these drawbacks in the prior art. This invention is a device that produces an equivalent capacitance across two output terminals, that can be linearly varied, that can be used in high-bandwidth applications, that is temperature stable, and that can be integrated in an integrated circuit.